1. Field of the Invention
The present invention relates to a method of mounting conductive balls on pads formed on a workpiece, and also relates to a device used in this method.
2. Related Art of the Invention
As a method of manufacturing a workpiece provided with bumps such as flip chips, there has been known a method in which solder balls or conductive balls are used. In this method, the conductive balls stored in a supply part are picked up at the lower surface of a suction head through vacuum suction, and are then mounted on pads on a workpiece. This method is advantageous since a large number of conductive balls can be mounted onto a workpiece in a batch.
Referring to FIGS. 9a to 9c which are views for explaining a conventional method of mounting conductive balls, a suction head is lowered and raised above a workpiece so as to mount conductive balls onto pads 5 on the workpiece 4. Referring to FIG. 9a, the conductive balls 1 are sucked up under vacuum to several suction holes 3 formed in the lower surface of the suction head 2. The inside of the suction head 2 is evacuated by a suction part (which is not shown) and accordingly, vacuum is effected in the inside of the suction head 2. The pads 5 on the workpiece 4 is coated thereover with flux 6. The workpiece 4 is, for example, a semiconductor chip or a substrate which is cut our from a wafer.
Next, as shown in FIG. 9b, the suction head 2 is lowered so as to land the conductive balls onto the pads 5 while the suction in the suction head 2 is stopped in order to release the vacuum suction to the conductive balls 1. As a result, the atmospheric air enters the inside of the suction head 2 through gaps defined between the conductive balls 1 and the suction holes (refer to the arrows indicated by broken lines in FIG. 9b), and accordingly, the atmospheric air is effected in the inside of the suction head 2. Thereafter, the suction head 2 is raised while the conductive balls 1 are left on the pads 5. Then, the workpiece 4 is sent to a heating furnace in which the conductive balls are heated so as to be melted, and then are cooled for solidification thereof. Thus, bumps are formed on the pads 5.
In the method as mentioned above, the suction head 2 is lowered so as to land the conductive balls 1 on the pads 5 while the vacuum suction in the inside of the suction head 2 is stopped, as shown in FIG. 9b. Accordingly, since the suction head 2 is raised after the atmospheric air having entered in the inside of the suction head 2 sufficiently breaks the vacuum condition in the inside of the suction head 2, the time tact for setting the conductive balls 1 on the pads 5 becomes excessively long, and accordingly, the productivity associated therewith is low.
In order to solve the above-mentioned problem, there has been known such a measure that the suction head 2 is vibrated by a vibrator (which is not shown) in conditions shown in FIGS. 9b and 9c in order to promote the separation of the conductive balls 1 from the suction holes 3. Even with this measure, a relatively long time tact is required, and further, there has be such a problem that the conductive balls 1 are not always surely separated from the suction holes 3.